In recent years, lithium ion batteries, nickel hydride batteries, and other secondary batteries are growing in importance as vehicle-mounted power supplies and as power supplies for personal computers and mobile phones. In particular, since lithium ion batteries are lightweight and capable of producing high energy density, they are expected to be favorably used as high-output vehicle-mounted power supplies. In a typical configuration of such a lithium secondary battery, a separator is interposed between a positive electrode and a negative electrode to prevent short circuit between the positive and negative electrodes. A polyolefin porous film in which pores are formed in a large number is used as the separator in order to secure ion permeability between the positive electrode and the negative electrode.
However, a polyolefin porous film becomes susceptible to thermal contraction or fracture when the temperature inside a battery rises and, as a result, an internal short circuit (short) may conceivably occur. In order to prevent such an occurrence of a short circuit, the formation of a heat-resistant porous layer on a surface of a separator is being evaluated. For example, Patent Literature 1 discloses a nonaqueous secondary battery separator in which a heat-resistant porous layer is formed on a surface of a polyolefin microporous membrane.